This application relates to inductive coupling system transformers and high frequency DC-DC converters.
The invention relates to an inductive coupling system comprising: a magnetizable core with a primary yoke (2) which is provided with a primary winding (4) for connecting an AC supply voltage (Vp) and a secondary yoke (6) which is provided with a secondary winding (8), which primary yoke (2) and secondary yoke (6) have corresponding end surfaces (10, 14; 12, 16) for magnetic energy transfer between the primary yoke (2) and the secondary yoke (6).
Such an inductive coupling system is known as a transformer, which may or may not form part of a DC-DC converter which operates at a high frequency and in which the primary and secondary yokes of the transformer core are rigidly disposed with respect to each other and are mechanically integral with each other. An example is the so-called xe2x80x9cpower plugxe2x80x9d, in which the mains voltage is converted by means of a DC-DC converter into a lower operating voltage which is not in direct electrical contact with the mains voltage.
Such an inductive coupling system is also known from contactless inductive charging systems for rechargeable appliances, such as electric toothbrushes, razors and mobile telephones. In this case, the primary and secondary yokes can be separated, the primary yoke being accommodated in a so-called xe2x80x9cstandxe2x80x9d and the secondary yoke being accommodated in the rechargeable appliance. The rechargeable appliance is placed back in the stand after use, such that the primary and secondary yokes are so positioned with respect to each other that the yokes and their windings form a transformer again.
In both the aforesaid cases, the relatively large air gap between the end surfaces of the yokes leads to an imperfect magnetic coupling between the primary part and the secondary part of the coupling system. In the case of fixed transformers, it may be the cost price and dimensional tolerance that causes this large air gap, and in the case of inductive charging systems, the main cause is the nature of the design of the stand and of the appliance. A consequence of the large air gap is that a substantial portion of the magnetic field lines that exit from the end surfaces of the primary yoke is not detected by the corresponding end surfaces of the secondary yoke. This leads to major wattless currents through the primary winding and to losses in the primary winding and in the electronic components that drive the primary winding.
A solution might be to increase the dimensions of the yokes so as to increase the magnetic coupling between the yokes, but this leads to an increased cost price on the one hand and to a limitation of the freedom of design on the other hand.
Accordingly, it is an object of the invention to provide an inductive coupling system which exhibits an improved magnetic coupling between the primary and the secondary parts of the coupling system.
In order to accomplish the above object, the inductive coupling referred to in the introduction is characterized in that said inductive coupling system comprises means for capacitive parallel compensation of a mutual self-inductance of the coupling system at the frequency of the primary AC voltage.
In the equivalent model of the inductive coupling system, the magnetic coupling between the primary and the secondary parts is represented by the mutual self-inductance. The poor magnetic coupling manifests itself as a low value of the mutual self-inductance in comparison with the primary leakage inductance. The capacitive parallel compensation provides a capacitance which is connected in parallel to the mutual self-inductance and which, together with the mutual self-inductance, forms a parallel resonance circuit that resonates at the frequency of the primary AC voltage. In the case of parallel resonance, the impedance of the parallel circuit is high and hardly any wattless current flows from and to the parallel circuit any more. The impeding influence of the air gap is considerably reduced in this manner, and consequently nearly all magnetic energy will still flow from the primary part to the secondary part of the coupling system without the dimensions of the yokes themselves being changed.
The capacitive parallel compensation is preferably realized in the form of an auxiliary winding which is arranged near at least one of the aforesaid end surfaces, to which auxiliary winding a capacitor is connected which resonates with the auxiliary winding at the frequency of the primary AC voltage.
Various advantageous configurations as claimed in the dependent claims are possible for placing one or more auxiliary windings on the yokes of the inductive coupling system, which yokes may be U-shaped or E-shaped.